Variable impedance device



Feb. 19, 1957 F. E. WELD ,782, 8

VARIABLE IMPEDANCE ,DEVICE Filed April 8, 1954 INVENTOR. FOSTER E. WE 0 ATTORNEYS United States atent VARIABLE IMPEDANCE DEVICE Foster E. Weld, Newton Highlands, Mass, assignor to The Gamewell Company, Newton Upper Falls, Mass a corporation of Massachusetts Application April 8, 1954, Serial No. 421,767

8 Claims. (Cl. 201-48) The present invention relates to variable impedance devices and more particularly to a novel construction for a rheostat, potentiometer or the like, wherein the impedance element is a winding of the toroidal type.

The invention has as its principal object the provision of a construction permitting easy and accurate assembly of the parts, especially in making the connections to the impedance element, while at the same time insuring that the electrical circuit is firmly connected and will so remain over a long period of service.

Many of the problems incident to the manufacture of potentiometers and the like arise from the increasing demands of the electrical arts for smaller size, coupled with the greatest possible precision and reliability of performance. The difiiculties of handling the parts and of accurate assembly increase when the unit size is reduced. It is desirable to reduce as far as practicable the element of hand labor which is required. Accordingly, it is a further object :of the invention to provide an assembly which can be readily put together with precision, yet requiring a minimum of effort, time and skill.

The increasing need for uniformity of product in the art, which is often difficult to satisfy especially in the manufacture of miniature components, gives rise to the still further object of providing an assembly having positive provision to insure precision and uniformity in the positioning of connections to the impedance element.

With the above and other objects in view, the features of the invention include the provision of a guide element or ring of insulating material as an integral part of the assembly. This ring has slots accurately machined therein. Spring contact Wires extend through the slots from the terminal block, and make connection with the impedance element and the movable brush contact which rests upon the impedance element. The slots are narrow in width, and the wires are thus accurately positioned in relation to the impedance element.

The contact wires are arranged to be bent back so as to bear upon the impedance winding with suitable pressure, and at positions accurately determined by the positions of the respective slots.

The brush contact is mounted upon a plate rotatably supported upon a shaft, the plate lying in a plane at right angles thereto. One of the wires extending through a slot in the guide element is arranged to bear slidably upon the surface of the plate.

In certain cases, the extremely small diameter of the contact wires creates a problem in providing means to connect the wires to the terminal block. According to a feature of this invention, the problem is solved by provision of ductile tubing of relatively small inner diameter which may be swaged onto an insulating terminal strip or block with the contact wires inserted, whereby the wires are secured within the tubing in the same operation.

Other features of the invention comprise certain arrangements, relationships and structural features which will be more clearly understood from the following dcscription, and from the claims.

In the drawings,

Fig. 1 is a side elevation in section of the preferred form of the invention;

Fig. 2 is an end elevation looking from the left in Fig. l; and

Fig. 3 is a fragmentary perspective view showing various parts cut away to illustrate the location of the contact wires in the guide ring.

Referring to the drawings, the body 2 of the potentiometer is a metallic element having a central bore, a portion 4 of reduced outer diameter having a standard thread to provide means for assembly of the unit with a nut upon a chassis, and a portion of enlarged diameter bored and counterbored to provide an inner annular shoulder 6. In the portion of larger inside diameter thus created, an elongated slot 8 is cut through the wall of the body. A shallow inner annular shoulder 10 is preferably provided to receive a suitable cover plate. A terminal block 12, fabricated of suitable insulating material and shaped as a cylindrical segment, is secured onto the body 2 by screws 14.

A shaft 16 is rotatably supported in the body 2. A portion 18 of the shaft is preferably relieved to reduce friction. Washers 20 are received over the shaft, and split retaining rings 22 of a conventional type are snapped into annular grooves in the shaft to prevent axial movement. A rotor bushing 24 of suitable insulating material is supported at the end of the shaft and is preferably cemented thereto as illustrated at 26. The bushing is relieved slightly at its end to receive a thin metal contact plate 28, which is preferably secured thereto by means of cement 36. The plate is preferably fabricated of spring bronze, and is silver clad on the right-hand side as viewed in Fig. l, which is in sliding engagement with a contact wire to be subsequently described. A brush consisting of a short piece of contact wire 32 is soldered or welded to the plate.

A guide element 34, which is formed in the shape of a ring and constructed of a suitable insulating material, is supported within the body 2, and rests against the shoulder 6 thereof. This ring has three thin slots 36., 38 and 40 machined therein in precise relation to one another. The slots 38 and 40 are preferably radial to the shaft axis, while the slot 36 is oifset in the manner illustrated in Fig. 2. The ring 34 is positioned so that these three slots are beneath the slot 8 in the body 2.

One end of the guide ring 34 is slightly relieved as illustrated at 42, Fig. 3. A relatively thin insulating strip 44 is received into the body 2, and fits into this relieved portion as shown in Fig. l. A toroidally-wound impedance element 46 is received into the body 2 against the guide ring 34, and is held out of contact with the body 2 by the insulating strip 44. The impedance elements, the strip 44 and the ring 34 are preferably held in place by an annular ring :of cement as illustrated at 48. The brush 32 rests with pressure upon the winding 46 when the unit is completely assembled.

The impedance element 46 is preferably constructed by winding suitably lacquered resistance wire about a ceramic toroidal ring 50. Obviously the ring 50 could be of other than true toroidal shape. For example, it might be extended into a cylinder, which would permit a greater length of wire to be accommodated for a given diameter. Also, the impedance element might be essentially inductive rather than resistive. The ends of the winding are open, as indicated by the short bare segment of the ring 50 near the top thereof in Fig. 2.

Three short lengths of suitable ductile tubing such as copper are swaged upon the terminal block 12 to form terminals 54. In the swaging operation ring-like por tions 56 are formed on each terminal above and below the terminal block 12. Each tube has a central bore into which has been received a length of noncorrosive contact wire of spring quality, such as gold clad spring bronze wire. Two :of the wires 58 and 66 are received into the slots 38 and 40 of the guide ring 3 5, respectively, and bent back so as to rest with pressure against the impedance element 46 adjacent the ends thereof. A wire62 passes through the offset slot 36, and is bent so as to bear slidably upon the inner surface of the contact plate 28.

If desired, two slots like the slot 36 may be provided, both terminated adjacent the same terminal 54, and two wires may be connected to the terminal 54, one passing through the slot 36, and the other passing through the other slot which would align with the contact plate symmetrically with the slot 36 but on the opposite side of the shaft. In this manner, the frictional and contact resistance of the wires would be the same for both directions of rotation of the shaft. However, in most applications a single brush produces satisfactory results.

The small diameter of the contact wires 58, 6t and 62 creates no particular problem, according to the present construction, since each is secured to the corresponding terminal simply by first inserting it into a short length of. tubing and then swaging the tubing onto the terminal block to form a terminal 54. The swaging operation sufiiciently reduces the inner diameter of the tubing to grip the wire tightly with a minimum danger of injury to the wire or of making an imperfect connection.

The assem ly of the complete potentiometer prefer ably begins with the foregoing operation joining the contact wires, terminals and terminal block. to form a subassembly. The succeeding steps are then easily accomplished with a minimum of skill, but with considerable accuracy and uniformity resulting from the described structure and arrangement of the parts.

First the shaft 15, preferably without the bushing 24, .iS received into the body 2, the washers are received over the shaft and the retaining rings 22 are snapped into lplace. Next, the guide ring 34 is received into the body against the shoulder 6 with the slots 36, 33 and beneath the elongated slot 3 in the body. Next, the terminal block sub-assembly is placed in position, and the contact Wires fed through the respective slots in the ring 34 asshown in Fig. 3. The terminal block is held in place by the screws 14.

Next, the insulating strip 44 is slipped into the recess provided by the relieved portion of the guide ring. In this manner, the strip 44 further confines each wire within its corresponding slot. This automatically eliminates slight differences in the alignment of the wires in the slots of ditierent units being assembled, which may have resulted from variations in the previously described steps. for example in the manual alignment of the terminals above the slots.

Next the resistance element is fitted in place with the ends of the toroidal winding falling within the angle subtended between the slots 38 and 40. The winding pref erably has a few extra turns at each end, so that the alignment need only be approximate. Since the positions of the contact wires bearing upon the ends of the winding are accurately determined by the slots 38 and 40 the total impedance therebetween will remain the same despite any differences in positioning of the winding, so long as the ends of the winding fall in the space between the slots 38 and ii). The winding, the strip 44 and the ring 34 are held in place by cement 43.

The contact wires 58 and which make contact with the winding are preferably bent back to bear upon the winding as it is fitted into place. In the drawings only a single bend per wire is illustrated, but in some embodiments each wire may have more than one bend. As shown in Fig. l, the ends of these wires are bent back so that the portions in contact with the winding 46 are constrained within the respective slots.

The plate 28 is preferably cemented to the rotor bushing 24- before assembly of these parts onto the shaft 16. The final step is to position the insulating bushing onto the shaft at a position in which the brush 32 bears with satisfactory pressure against the winding. Previously, the wire 62 has been bent forward so as to make contact with the bearing surface of the plate 7.8 when it is placed in position.

It will thus be seen that a potentiometer assembly has been provided which comprises parts of simple, easily fabricated shape. These parts fit together with a minimum of effort and skill; yet, in all essential respects the completed assembly is precise as to the spacing between the end connections of the winding, and all electrical contacts are secure. The precision with which the total impedance between the end connections is determined depends, first, upon the precision with which the winding is wound about the ring 50, including the uniformity of resistance per unit length of the wire used and the uniformity of spacing of the turns, and second, upon the precision with which the slots 38 and 40 are machined with respect to one another. Thus, in the illustrated embodiment the siots 3% and it are machined in precise angular relationship upon radii extending from the axis of the toroidal winding, and the positions of the respective wires 58 and 64 upon the impedance element are therefore precisely determined. Variations in manual alignment of the parts during assembly are substantially eliminated, or are of negligible importance and do not adversely affect the precision or uniformity of assembly. The above-described assembly is especially suitable for miniature potentiometers for the above reasons, and also by virtue of the simplified method of swaging the wires onto the terminals 5%, and the easily fabricated shapes of the various parts. The unit can also be constructed as a rheostat, in which case one of the contact wires 53 or 60 and its associated terminal 54 can be eliminated.

While the invention has been described with reference to a preferred embodiment, it will be recognized that many variations in design, dimension, and details of construction, in addition to those previously mentioned, may be effected in accordance with the methods and skills presently known to the art without departing from the spirit or scope of the invention.

Having thus described the invention, I claim:

.l. A variable impedance assembly having, in combination, a body, a shaft rotatably supported in the body, a substantially toroidal impedance element supported by the body about the shaft axis. a contact plate secured to the shaft having a brush contact bearing upon the impedance, a guide element supported by the body and provided with a number of thin slots, a terminal supported by the body adjacent each slot, and a spring contact wire secured to each terminal and extending into the adjacent slot, one wire being constrained by its slot and bent so as to bear resiliently at a precisely determined position upon said impedance with its constraining slot determining the position in which it bears on said impedance, and another wire being bent so as to bear slidably upon said plate.

2. A potentiometer assembly having, in combination, a body, a shaft rotatably supported in the body, a substantially toroidal impedance element supported by the body about the shaft axis, a contact plate secured to the shaft having a brush contact bearing upon the impedance, a guide clement supported by the body, said guide having a pair of thin slots positioned in predetermined relation to the impedance element and a third slot positioned in predetermined relation to the contact plate, a terminal supported by the body adjacent each slot, and a spring contact wire secured to each terminal and extending into the adjacent slot, a pair of said wires being constrained by their respective slots and bent so as to bear resiliently at precisely determined positions upon said impedance, said last-mentioned positions being determined by the respective slots, and a third wire being bent so as to bear slidably upon said plate.

3. A variable impedance assembly having, in combination, a 'body, a shaft rotatably supported in the body, a substantially toroidal impedance element supported by the body about the shaft axis, a contact plate secured to the shaft having a brush contact bearing upon the impedance, a guide element supported by the body and provided with a number of thin slots, a terminal provided With a bore adjacent each slot, and a spring contact Wire secured Within the bore of each terminal and extending into the adjacent slot, one wire being constrained by its slot and bent so as to bear resiliently at a precisely determined position upon said impedance, said position being determined by the slot, and another wire being bent so as to bear slidably upon said plate.

4. A potentiometer assembly having, in combination, a metallic body having a recess in an end thereof, a shaft rotatably supported in the body and extending into the recess, a substantially toroidal resistance element supported within the recess, a contact plate insulatedly secured to the shaft and provided with a brush contact bearing upon the impedance, an insulating guide ring supported within the recess and provided with a number of thin slots machined in precise relative relationship, a terminal insulatedly supported by the body adjacent each slot, and a spring contact wire secured to each terminal and extending into the adjacent slot, one wire being constrained by its slot and bent so as to bear resiliently at a precisely determined position upon said impedance opposite to said contact plate, the position of contact of the Wire on the impedance being determined by its constraining slot, and another wire being bent so as to bear slidably upon said plate.

5. A variable impedance assembly having, in combination, a body, a shaft rotatably supported in the body, a substantially toroidal impedance element supported by the body about the shaft axis, a contact plate secured to the shaft having a brush contact bearing upon the impedance, a terminal on the body, means connected with said terminal and slidably engaged with said plate, the body having an insulating section with a thin slot therein, a terminal supported by the body adjacent said slot, and a spring contact wire secured to said last-mentioned terminal and extending into the slot, said wire being constrained by the slot and bent so as to bear resiliently at a precisely determined position upon said impedance, the position of contact of the wire on the impedance being determined by the slot.

6, A potentiometer assembly having, in combination, a body, a shaft rotatably supported in the body, a substantially toroidal impedance element supported by the body about the shaft axis, a contact plate secured to the shaft having a brush contact bearing upon the impedance, a terminal on the body, means connected With said terminal and slidably engaged with said plate, the body having an insulating section with two slots therein, a terminal supported by the body adjacent each slot, and a spring contact wire secured to each of said last-mentioned terminals and extending into the slots, said wires being constrained by their respective slots and bent so as to bear resiliently upon said impedance at relative positions determined precisely by the relative displacement of the slots in said section of the body.

7. The assembly according to claim 5 in which the portion of the Wire in contact with the impedance is constrained Within the slot.

8. A variable impedance assembly having in combination, a body, a shaft rotatably supported in the body, a substantially toroidal impedance element supported by the body about the shaft axis, a contact plate secured to the shaft having a brush contact bearing upon the impedance, a terminal on the body, means connected with said terminal and slidably engaged with said plate, the body having an insulating section with a thin slot therein, a terminal supported by the body adjacent the slot and having a bore, and a spring contact Wire passing through the bore and extending into the slot, the wire being constrained by the slot and bent so as to bear resiliently at a precisely determined position upon said impedance, the position of contact of the Wire on the impedance being determined by the slot and the terminal being swaged to secure it onto the body and to grip the portion of the wire within the bore.

References Cited in the file of this patent FOREIGN PATENTS 659,778 Great Britain Oct. 24, 1951 

